1. Field of Invention
This invention relates to metallic orthopedic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant coating of blue-black zirconium oxide, black zirconium oxide or zirconium nitride. These coatings are especially useful on the portions of these prostheses which bear against softer surfaces such as body tissue surfaces.
In the preferred oxidation process by which a zirconium oxide coating is produced, the associated increase in surface oxygen content and hardness increases the strength of the metal substrate and improves the fatigue properties of the implant. The oxide or nitride surfaces, being ceramic, do not release potentially harmful metal ions into the body and are not subject to galvanic corrosion in vivo. Further, the ceramic surfaces have enhanced hemocompatibility.
2. Description of the Related Art
Orthopedic implant materials must combine high strength, corrosion resistance, low friction and wear, and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient is relatively young because it is desirable that the implant should function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. 316L stainless steel, chrome-cobalt-molybdenum alloys and more recently titanium alloys have proven to be suitable materials for the fabrication of load-bearing prostheses.
One of the variables affecting the longevity of load-bearing implants such as hip-joint implants is the rate of wear of the articulating surfaces and long-term effects of resultant metal ion release. Such implants may be in the form of a surface replacement which is attached over the neck region of the original femur or a hip-joint prosthesis which has a femoral head attached to a stem portion which is fixed within the proximal shaft of the femur. A typical hip-joint prosthesis for total hip replacements includes a stem, a femoral head, and an acetabular cup against which the femoral head articulates. In the case of hemiarthroplasty, the natural acetabulum is retained so that the prosthetic femoral head articulates against natural body cartilage which is much softer than either metals or ceramics used to fabricate femoral heads. In total hip replacement, wear of either or both of the articulating surfaces results in an increasing level of wear particulates and "play" between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue reaction leading to bone resorption, and ultimately the joint must be replaced.
Generally, in hip hemiarthroplasty the upper (proximal) portion of the natural femur is replaced with a surface replacement or a prosthesis bearing a femoral head for cooperating slidingly with the cartilaginous material in the acetabulum of the natural hip, which is not replaced with the usual prosthetic acetabular cup. This softer cartilage tissue is then subjected to sliding wear induced by the action of a hard unipolar femoral head prosthesis.
The rate of wear of acetabulum cartilage and femoral head surfaces after hemiarthroplasty is dependent upon a number of factors which include the relative hardness and surface finish of the materials which constitute the femoral head, the susceptibility of the head material to ionization and galvanic corrosion, the frictional coefficient between the materials of the head, and cartilage, the load applied and the stresses generated at the articulating surfaces, among other factors. The most common material currently used in the fabrication of hemiarthroplasty hip-joint implants include femoral heads of stainless steel or cobalt or titanium alloys and femoral heads of polished alumina. Of the other factors which influence the rate of wear of hemiarthroplasty hip-joint implants, the most significant are patient weight and activity level.
In the case of a metallic unipolar femoral head, sliding action of femoral head against acetabulum cartilage may gradually erode passive oxide film on its surface and expose body tissue to metal which could lead to release of metal ions and adverse tissue reaction. A ceramic unipolar femoral head, on the other hand, has undesirably a much higher modulus than even metals and is not as impact resistant or shock absorbent as bone or metal. Thus, ceramics while producing lower friction and being usually more biocompatible than metals, suffer significant drawbacks with respect to shock absorbance and impact resistance.
U.S. Pat. No. 4,145,764 to Suzuki et al recognized that while metal prostheses have excellent mechanical strength they tend to corrode in the body by ionization. Suzuki et al also recognized the affinity between ceramics and bone tissue, but noted that ceramic prostheses are weak on impact resistance. Suzuki et al therefore proposed a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous ceramic coating which would allow the ingrowth of bone spicules into the pores. This combination, it was said, would provide both the mechanical strength of metals and the biocompatibility of ceramics. The application of ceramic coatings to metal substrates often results in non-uniform, poorly-bonded coatings which tend to crack due to the differences in thermal expansion and hardness mismatch between the ceramic and the underlying metal substrate. Furthermore, such coatings are relatively thick (50-300 microns) and relatively porous, and since the bond between the metal and the ceramic coating is often weak there is always the risk of galling or separation of the ceramic coating.
U.S. Pat. No. 3,677,795 to Bokros is directed to the application of a carbide coating over a metallic prosthetic device. This method of forming the carbide coating requires that the prosthesis be heated to temperatures of at least about 1350.degree. C. in a reaction chamber through which a hydrocarbon gas such as propane or butane flows. The method is said to produce a prosthetic device which has "excellent compatibility with body tissue and is non-thrombogenic." Bokros does not address the issues of friction, heating, creep and wear of orthopedic implant bearing surfaces, or changes induced in the mechanical properties of the underlying metal due to this high-temperature treatment. Carbonaceous coatings are much less ionic than oxide ceramic coatings (particularly ZrO.sub.2 and Al.sub.2 O.sub.3) and are thus less wettable and produce higher friction.
U.S. Pat. No. 3,643,658 to Steinemann is directed to titanium implants coated with titanium oxide, nitride, carbide or carbonitride to prevent corrosion and abrasion of the implant. These coatings are also said to protect the titanium implant from fretting wear. The coatings vary in thickness from 0.08 microns to about 0.15 microns. Titanium oxide forms naturally on titanium and titanium alloy in ambient conditions. Titanium oxide coatings are not well attached, are not dense and adherent, and are not effective as protective coatings to prevent metal ion release into the body. The oxide film is thin (0.5-7 nm) to a point where it is transparent to the naked eye and is similar to the protective passive oxide layers in cobalt alloys and stainless steels formed primarily from the chromium content. These types of natural passive oxide layers formed under ambient conditions or by nitric acid passivation (usually used for metal orthopaedic implants) can easily abrade off from motion and contact against surrounding material, even soft polymeric materials or body tissue. Under these conditions, metal ions are released into the body environment. For the case of titanium and titanium alloys, amorphous titanium monoxide (TiO) forms at room temperature with small quantities of Ti.sub.3 O.sub.5. The oxide is easily disturbed in a saline environment resulting in repassivation of an intermediate oxide 3Ti.sub.2 O.sub.3.4TiO.sub.2. Formation of the higher oxide, TiO.sub.2 (anatase) and Ti.sub.2 O occur at higher oxidation temperatures. However, under fretting conditions (with adjacent bone, bearing against polyethylene, and particularly against metal as in the case for bone screws in bone plates, etc.) all forms of normal passivated, and even high-temperature (350.degree. C.) surface anodized titanium oxide films provide little, if any, protection from spalling of the oxide and subsequent fretting of the metal substrate. Relatively thicker coatings using high current-density anodizing also provide little anti-fretting protection due to the poor adherence of the loose powdery films. In general, titanium oxide films are ineffective against fretting conditions because of their poor strength and attachment.
A totally inert, abrasion resistant monolithic ceramic may be ideal for eliminating fretting and metal ion release. For example, zirconium dioxide (ZrO.sub.2) and alumina (Al.sub.2 O.sub.3) have been shown to be highly inert, low friction, biocompatible implant materials. These ceramics have been in use recently as monolithic alumina or zirconium dioxide femoral heads in total hip replacements. Both materials are hard, dense, biocompatible, lubricous, and sufficiently strong. Importantly, when polished, the highly ionic, wettable ceramic bearing surface, articulating against ultra high molecular weight polyethylene (UHMWPE), not only significantly reduces the frictional moment against the UHMWPE cup but also greatly reduces the rate of wear of the UHMWPE. Similarly, monolithic ceramic femoral heads have been implanted after hemiarthroplasty so that ceramic surfaces cooperate slidingly against natural cartilage of the acetabulum. However, solid ceramics have high modulus and low shock absorbance so that ceramic implants stress shield surrounding bones leading to bone decalcification and resorption while at the same time transmitting relatively higher shock forces to these bones. Beneficially, during articulation, no metal ions or micron-size fretted particulates from the ceramic are produced. Thus, these ceramics have certain advantages over cobalt, stainless steel, and titanium alloy bearing surfaces but also have significant disadvantages.
There exists a need for a hemiarthroplasty orthopedic implant having low friction, highly wear and corrosion resistant load bearing surfaces which may be implanted for the lifetime of the recipient. Further, the bearing surfaces should not cause wear and damage to the natural cartilage of the acetabulum or excessive stress shielding of bone that results in bone resorption and decalcification. Thus, the hemiarthroplasty implant should desirably have a modulus of elasticity closer to that of bone than monolithic ceramics while at the same time possessing low friction and not being susceptible to metal ion release, found in the case of metal implants.